This invention relates to a carrier for an image developer, to an electrostatic latent image developer and to an image forming apparatus by electrophotography, electrostatic recording or electrostatic printing.
In electrophotography, an electrostatic latent image formed on a photosensitive medium is developed by a developer containing a toner. A two-component developer including a carrier and a toner is widely used. Since the amount of the toner decreases during repeated use of the developer, it is necessary to replenish the developer with an amount of the toner in order to obtain images with a constant image density in a stable manner.
One known carrier for such a developer is composed of a core material covered with a resin coating. The resin coating is used for various purposes such as prevention of the formation of a toner film on the core material, provision of a smooth, non-abrasive surface, prevention of surface oxidation, prevention of moisture absorption, improvement of service life, and control of the polarity and electric charge.
However, known carriers still cause problems of adhesion of toner on the surface of the carrier (spent toner problem) and problems of wear of the resin coating during repeated use of the developer, which result in a change of surface resistivity of the carrier and deterioration of image quality. Namely, since a two-component developer is always subjected to stresses of collisions between carrier particles, between carrier particles and toner particles and between carrier particles and inside wall of a development box throughout the service time, toners are apt to firmly bond to the carrier surface. Such a spent toner is not transferred to an electrostatically charged latent image-bearing surface and thus causes deterioration of the image quality.
JP-A-H9-160304 discloses a carrier for an electrostatic latent image developer for electrophotography, including a carrier core material, and a coating layer covering the carrier core material and containing a resin and electrically conductive powder, wherein the average particle diameter B (xcexcm) of the powder and the thickness A (xcexcm) of the coating layer satisfy the following condition:
Axe2x89xa6Bxe2x89xa6A+3 xcexcm.
In this carrier, the powder is used as a conductor for preventing an increase of the resistivity of the carrier and has a specific resistance of not greater than 1010 xcexa9xc2x7cm. Further, the conductive powder is used in an amount of 0.01-33.3% based on the weight of the coating layer (or 0.01-50% based on the resin of the coating layer).
It is an object of the present invention to provide a carrier for a two-component developer for electrophotography, which includes a core material covered with a resin coating layer and which is free of spent toner problems.
Another object of the present invention is to provide a carrier of the above-mentioned type which is free of problems of wear of the resin layer.
It is a further object of the present invention to provide a carrier of the above-mentioned type which can produce high quality images for a long period of time.
In accomplishing the foregoing objects, the present invention provides a carrier for an image developer for electrophotography, comprising a core material, and a coating layer covering said core material and containing a binder and a powder having an average particle diameter of D xcexcm and a specific resistance of at least 1012 xcexa9xc2x7cm, said coating layer having a thickness of h xcexcm, wherein the ratio D/h is greater than 1:1 but less than 5:1.
The term xe2x80x9caverage particle diameterxe2x80x9d used in the present specification refers to weight average particle diameter.
The carrier is used together with a toner having a coloring agent dispersed in a binder resin as a two-component developer.
The developer is contained in a container for storage and transportation. In use, the container is mounted on an image forming apparatus such as a printer or a copying machine.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.
The according to the present invention comprises a core material, and a coating layer covering the core material.
Any conventionally employed core material for two-component developers may be used for the purpose of the present invention. Illustrative of suitable core materials are ferrite, magnetite, iron, nickel. The core material preferably has an average particle diameter of at least about 20 xcexcm for reasons of prevention of deposition of the carrier on a latent image-bearing surface of, for example, a photoconductive drum. For reasons of image quality, The core material preferably has an average particle diameter of not greater than about 100 xcexcm.
The coating layer of the core material has a thickness of h xcexcm and contains a binder and a powder having an average diameter of D xcexcm. It is important that the ratio D/h should be greater than 1:1 but less than 5:1 in order to ensure both satisfactory anti-spent toner property and satisfactory anti-wear property. When the D/h ratio is 1 or less, the powder is buried within the coating layer and cannot guard the coating layer from the friction, shock, abrasion and stress by the contact and collision of the carrier particles. To large a D/h ratio of 5 or more, on the other hand, is undesirable, because the powder cannot be tightly secured by the coating layer. The D/h ratio is preferably 1-4.
It is also important that the powder incorporated into the coating layer have a specific resistance of at least 1012 xcexa9xc2x7cm. Because of the high specific resistance, even when the powder secured to the core material by the binder is exposed on the surface of the carrier, leakage of charges does not occur. Thus, throughout its long service time, the carrier shows satisfactory charging amount and stable chargeability. When the specific resistance of the powder is less than 1012 xcexa9xc2x7cm, leakage of the charge on the carrier occurs through the powder.
Any powder may be used for the purpose of the present invention as long as the specific resistance thereof is at least 1012 xcexa9xc2x7cm. Surface-treated or non-treated inorganic oxide powder may be used. The surface treatment may be to impart hydrophobicity to the powder. Illustrative of suitable powder are alumina (non-treated or surface-treated) and silica (non-treated or surface-treated). The powder preferably has an average particle diameter of about 0.05 to about 5 xcexcm.
Any binder customarily used for coating a core material of carriers may be employed in the present invention. Examples of the binder include polystyrene resins, polyacryllc resins, polymethacrylic resins, polyolefin resins, polyamide resins, polycarbonate resins, polyether resins, polysulfinic acid resins, polyester resins, epoxy resins, polybutyral resins, urea resins, urethane-urea resins, silicone resins, teflon resins, copolymers thereof including block copolymers and graft copolymers, and mixtures thereof.
A binder resin obtained by crosslinking an acrylic resin with an amino resin is particularly suitably used for reasons of improved durability and service life of the carrier. The acrylic resin preferably has a glass transition point Tg of 20-100xc2x0 C., more preferably 25-90xc2x0 C., most preferably 25-80xc2x0 C., since the coating layer can exhibit suitable elasticity and can absorb the shock of collision of the carrier during use. The amino resin for crosslinking the acrylic resin may be, for example, a guanamine resin or a melamine resin.
The amount of the powder in the coating layer is preferably 50-95% by weight, more preferably 60-90% by weight, most preferably 70-90% by weight. Too large an amount of the powder in excess of 95% by weight will cause reduction of chargeability of the carrier as well as release of the powder from the carrier during use. An amount of the powder below 50% by weight will be insufficient to provide desired anti-spent toner and anti-wear properties.
The coating layer preferably has a thickness h of 0.05-1.0 xcexcm, more preferably 0.06-0.8 xcexcm, most preferably 0.1-0.7 xcexcm.
The coating layer of the carrier may include one or more additives such as a resistivity controlling agent (e.g. carbon black) and an acid catalyst. The acid catalyst which may be, for example, a compound having an alkyl group or a reactive group such as a methylol group, an imino group or both methylol and imino groups, serves to function as a promotor for crosslinking of an acrylic resin with an amino resin.